DON'T MERGE: User plugins proposal and prototype

PLUGIN-DESIGN.md

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+# User Plugins Design Proposal
+
+## Background
+
+This is a design proposal for the long-standing [#55](https://github.com/rojo-rbx/rojo/issues/55)
+desire to add user plugins to Rojo for things like source file transformation and instance tree
+transformation.
+
+As discussed in [#55](https://github.com/rojo-rbx/rojo/issues/55) and as initially explored in
+[#257](https://github.com/rojo-rbx/rojo/pull/257), plugins as Lua scripts seem to be a good starting
+point. This concept reminded me of Rollup.js plugins. [Rollup](https://rollupjs.org/guide/en/) is a
+JS bundler which performs a similar job to Rojo in the JS world by taking a number of source files
+and producing a single output bundle. [Rollup
+plugins](https://rollupjs.org/guide/en/#plugins-overview) are written as JS files, and passed to the
+primary Rollup config file. Rollup then calls in to plugin "hooks" at special times during the
+bundling process. I have found Rollup plugins to be an excellent interface for both plugin
+developers and end-users and therefore I have based this proposal on their API.
+
+This proposal attempts to take advantage of the existing "middleware" system in Rojo, and give
+plugins the opportunity to:
+
+-   Override the default choice for which internal middleware should be used for a given file path
+-   Transform the contents of a file before consuming it
+
+## Project file changes
+
+Add a new top-level field to the project file format:
+
+-   `plugins`: An array of [Plugin Descriptions](#plugin-description).
+    -   **Optional**
+    -   Default is `[]`
+
+### Plugin description
+
+Either a `String` value or an object with the fields:
+
+-   `source`: A filepath to the Lua source file of the plugin or a URL to a GitHub repo, optionally
+    followed by an `@` character and a Git tag.
+    -   **Required**
+-   `options`: Any JSON dictionary. The options that will be passed to the plugin.
+    -   **Optional**
+    -   Default is `{}`
+
+In the case that the value is just a `String`, it is interpreted as an object with the `source`
+field set to its value, and `options` set to its default.
+
+### Example
+
+```json
+{
+    "name": "ProjectWithPlugins",
+    "plugins": [
+        "local-plugin.lua",
+        "github.com/owner/remote-plugin-from-tag@v1.0.0",
+        "github.com/owner/remote-plugin-from-head",
+        { "source": "plugin-with-options.lua", "options": { "some": "option" } }
+    ],
+    ...
+}
+```
+
+## Plugin scripts
+
+Plugin scripts should return a `CreatePluginFunction`:
+
+```luau
+-- Types provided in luau format
+
+type PluginInstance = {
+  name: string,
+  projectDescription?: (project: ProjectDescription) -> (),
+  syncStart?: () -> (),
+  syncEnd?: () -> (),
+  resolve?: (id: string) -> string,
+  middleware?: (id: string) -> string,
+  load?: (id: string) -> string,
+}
+
+-- TODO: Define properly. This is basically just the JSON converted to Lua
+type ProjectDescription = { ... }
+
+type CreatePluginFunction = (options: {[string]: any}) -> PluginInstance
+```
+
+In this way, plugins have the opportunity to customize their hooks based on the options provided by
+the user in the project file.
+
+## Plugin instance
+
+-   `name`
+    -   **Required**: The name of the plugin that will be used in error messages, etc.
+-   `projectDescription(project: ProjectDescription) -> ()`
+    -   **Optional**: Called with a Lua representation of the current project description whenever
+        it has changed.
+-   `syncStart() -> ()`
+    -   **Optional**: A sync has started.
+-   `syncEnd() -> ()`
+    -   **Optional**: A sync has finished.
+-   `resolve(id: string) -> string`
+    -   **Optional**: Takes a file path and returns a new file path that the file should be loaded
+        from instead. The first plugin to return a non-nil value per id wins.
+-   `middleware(id: string) -> string`
+    -   **Optional**: Takes a file path and returns a snapshot middleware enum to determine how Rojo
+        should build the instance tree for the file. The first plugin to return a non-nil value per
+        id wins.
+-   `load(id: string) -> string`
+    -   **Optional**: Takes a file path and returns the file contents that should be interpreted by
+        Rojo. The first plugin to return a non-nil value per id wins.
+
+## Plugin environment
+
+The plugin environment is created in the following way:
+
+1. Create a new Lua context.
+1. Add a global `rojo` table which is the entry point to the [Plugin library](#plugin-library)
+1. Initialize an empty `_G.plugins` table.
+1. For each plugin description in the project file:
+    1. Convert the plugin options from the project file from JSON to a Lua table.
+    1. If the `source` field is a GitHub URL, download the plugin directory from the repo with the
+       specified version tag (if no tag, from the head of the default branch) into a local
+       `.rojo-plugins` directory with the repo identifier as its name. It is recommended that users
+       add `.rojo-plugins` to their `.gitignore` file. The root of the plugin will be called
+       `main.lua`.
+    1. Load and evaluate the file contents into the Lua context to get a handle to the
+       `CreatePluginFunction`
+    1. Call the `CreatePluginFunction` with the converted options to get a handle of the result.
+    1. Push the result at the end of the `_G.plugins` table
+
+If at any point there is an error in the above steps, Rojo should quit with an appropriate error
+message.
+
+## Plugin library
+
+Accessible via the `rojo` global, the plugin library offers helper methods for plugins:
+
+-   `toJson(value: any) -> string`
+    -   Converts a Lua value to a JSON string.
+-   `readFileAsUtf8(id: string) -> string`
+    -   Reads the contents of a file from a file path using the VFS. Plugins should always read
+        files via this method rather than directly from the file system.
+-   `getExtension(id: string) -> boolean`
+    -   Returns the file extension of the file path.
+-   `hasExtension(id: string, ext: string) -> boolean`
+    -   Checks if a file path has the provided extension.
+
+## Use case analyses
+
+To demonstrate the effectiveness of this API, pseudo-implementations for a variety of use-cases are
+shown using the API.
+
+### MoonScript transformation
+
+Requested by:
+
+-   @Airwarfare in [#170](https://github.com/rojo-rbx/rojo/issues/170)
+-   @dimitriye98 in [#55](https://github.com/rojo-rbx/rojo/issues/55#issuecomment-402616429) (comment)
+
+```lua
+local parse = require 'moonscript.parse'
+local compile = require 'moonscript.compile'
+
+return function(options)
+  return {
+    name = "moonscript",
+    middleware = function(id)
+      if rojo.hasExtension(id, 'moon') then
+        if rojo.hasExtension(id, 'server.moon') then
+          return 'lua_server'
+        elseif rojo.hasExtension(id, 'client.moon') then
+          return 'lua_client'
+        else
+          return 'lua_module'
+        end
+      end
+    end,
+    load = function(id)
+      if rojo.hasExtension(id, 'moon') then
+        local contents = rojo.readFileAsUtf8(id)
+
+        local tree, err = parse.string(contents)
+        assert(tree, err)
+
+        local lua, err, pos = compile.tree(tree)
+        if not lua then error(compile.format_error(err, pos, contents)) end
+
+        return lua
+      end
+    end
+  }
+end
+```
+
+### Obfuscation/minifier transformation
+
+Requested by:
+
+-   @cmumme in [#55](https://github.com/rojo-rbx/rojo/issues/55#issuecomment-794801625) (comment)
+-   @blake-mealey in [#382](https://github.com/rojo-rbx/rojo/issues/382)
+
+```lua
+local minifier = require 'minifier.lua'
+
+return function(options)
+  return {
+    name = "minifier",
+    load = function(id)
+      if rojo.hasExtension(id, 'lua') then
+        local contents = rojo.readFileAsUtf8(id)
+        return minifier(contents)
+      end
+    end
+  }
+end
+```
+
+### Markdown to Roblox rich text
+
+```lua
+-- Convert markdown to Roblox rich text format implementation here
+
+return function(options)
+    return {
+        name = 'markdown-to-richtext',
+        middleware = function(id)
+            if rojo.hasExtension(id, 'md') then
+              return 'json_model'
+            end
+        end,
+        load = function(id)
+            if rojo.hasExtension(id, 'md') then
+              local contents = rojo.readFileAsUtf8(id)
+
+              local frontmatter = parseFrontmatter(contents)
+              local className = frontmatter.className or 'StringValue'
+              local property = frontmatter.property or 'Value'
+
+              local richText = markdownToRichText(contents)
+
+              return rojo.toJson({
+                ClassName = className,
+                Properties = {
+                  [property] = richText
+                }
+              })
+            end
+        end
+    }
+end
+```
+
+### Load custom files as StringValue instances
+
+Requested by:
+
+-   @rfrey-rbx in [#406](https://github.com/rojo-rbx/rojo/issues/406)
+-   @Quenty in [#148](https://github.com/rojo-rbx/rojo/issues/148)
+
+```lua
+return function(options)
+    options.extensions = options.extensions or {}
+
+    return {
+        name = 'load-as-stringvalue',
+        middleware = function(id)
+            local idExt = rojo.getExtension(id)
+            for _, ext in next, options.extensions do
+                if ext == idExt then
+                    return 'json_model'
+                end
+            end
+        end,
+        load = function(id)
+            local idExt = rojo.getExtension(id)
+            for _, ext in next, options.extensions do
+                if ext == idExt then
+                    local contents = rojo.readFileAsUtf8(id)
+                    local jsonEncoded = contents:gsub('\r', '\\r'):gsub('\n', '\\n')
+
+                    return rojo.toJson({
+                      ClassName = 'StringValue',
+                      Properties = {
+                        Value = jsonEncoded
+                      }
+                    })
+                end
+            end
+        end
+    }
+end
+```
+
+```json
+// default.project.json
+{
+  "plugins": [
+    { "source": "load-as-stringvalue.lua", "options": { "extensions": {"md", "data.json"} }}
+  ]
+}
+```
+
+### File system requires
+
+Requested by:
+
+-   @blake-mealey in [#382](https://github.com/rojo-rbx/rojo/issues/382)
+
+```lua
+-- lua parsing/writing implementation here
+
+return function(options)
+  local project = nil
+
+  return {
+    name = "require-files",
+    projectDescription = function(newProject)
+      project = newProject
+    end,
+    load = function(id)
+      if rojo.hasExtension(id, 'lua') then
+        local contents = rojo.readFileAsUtf8(id)
+
+        -- This function will look for require 'file/path' statements in the source and replace
+        -- them with Roblox require(instance.path) statements based on the project's configuration
+        -- (where certain file paths are mounted)
+        return replaceRequires(contents, project)
+      end
+    end
+  }
+end
+```
+
+<!-- ### Remote file requires
+
+This might be a terrible idea tbh (but still cool).
+
+```lua
+-- this pseudo-implementation doesn't really make sense atm as resolve would never be called with a
+-- URL like this
+
+-- download/caching implementation inline here
+
+return function(options)
+  return {
+    name = "remote-require",
+    resolve = function(id)
+      if id:match('^https?://.*%.lua$') then
+        local cachedId = fromCache(id)
+        return cachedId or nil
+      end
+    end,
+    load = function(id)
+      if id:match('^https?://.*%.lua$') then
+        local cachedId = downloadAndCache(id)
+        local file = io.open(cachedId, 'r')
+        local source = file:read('a')
+        file:close()
+        return source
+      end
+    end
+  }
+end
+``` -->
+
+## Implementation priority
+
+This proposal could be split up into milestones without all the features being present till the end
+to simplify development. Here is a proposal for the order to implement each milestone:
+
+1. Loading plugins from local paths
+1. Minimum Rojo plugin library (`readFileAsUtf8`)
+1. Calling hooks at the appropriate time
+    1. Start with `middleware`, `load`
+    1. Add `syncStart`, `syncEnd`, `resolve`
+    1. Add `projectDescription`
+1. Full Rojo plugin library
+1. Loading plugins from remote repos